In general, a lower traveling structure of a tracked vehicle such as a hydraulic excavator is constituted by including a track frame having left and right side frames, a traveling device provided on one end side of each side frame, an idler wheel provided the other end side of each side frame, and a crawler belt wound between a drive wheel (sprocket) provided on the traveling device and the idler wheel.
The traveling device of the hydraulic excavator is normally constituted by a hydraulic motor which is a rotating source and a reduction device which reduces and outputs a speed of rotation of the hydraulic motor. The reduction device is constituted by a fixed-side housing accommodating the rotating source having an output shaft on which a female spline part is provided, a rotating-side housing rotatably provided with respect to the fixed-side housing and driven by the rotating source, a first-stage planetary gear reduction mechanism which is accommodated in the rotating-side housing and reduces the speed of the rotation of the rotating source, and a second-stage planetary gear reduction mechanism located in the rotating-side housing and disposed between the rotating source and the first-stage planetary gear reduction mechanism and reducing the speed of the rotation of the first-stage planetary gear reduction mechanism and rotating the rotating-side housing.
The first-stage planetary gear reduction mechanism is constituted by the rotational shaft on which a male spline part connected with the female spline part of the output shaft is provided on one side in an axial direction, a first sun gear provided on the other side in the axial direction of the rotational shaft, a plurality of first planetary gears meshed with the first sun gear and an internal gear provided on an inner peripheral side of the rotating-side housing and revolving while rotating around the first sun gear, and a first carrier rotatably supporting each of the first planetary gears. The second-stage planetary gear reduction mechanism is constituted by a cylindrical second sun gear connected to the first carrier and having a through hole through which the rotational shaft is inserted, a plurality of second planetary gears meshed with the second sun gear and internal gears provided on the inner peripheral side of the rotating-side housing and rotating the rotating-side housing by rotating around the second sun gear, and a second carrier mounted on the fixed-side housing in a non-rotation state and rotatably supporting each of the second planetary gears (see Patent Document 1, Patent Document 2, and Patent Document 3, for example).